Studies on this project are directed toward acquiring basic information about the epidemiology and prophylaxis of human infections caused by disease agents of particular interest to the Rocky Mountain Laboratory. Although conventional epidemiologic statistical and diagnostic laboratory procedures are generally used, new laboratory techniques potentially useful for epidemiologic research are tried on occasion. The project is largely opportunistic in that investigation is channeled according to the particular interests of the investigator at the time and the circumstances surrounding the specific disease occurrence. During the past year, activities in the project involved elucidation of the mechanism of the unique association of Colorado tick fever virus and erythrocytes during acute infection; and the importance of prolonged cell-associated viremia in blood donations. Geographic distribution of human disease in Montana was characterized in serosurveys and a tick hemolymph test for identification of Colorado tick fever antigen by immunofluorescence was developed. In collaboration with the North Carolina Division of Health Services several new serodiagnostic procedures for Rocky Mountain spotted fever are being evaluated. They include micro- immunofluorescence, microagglutination and hemagglutination tests, all of which appear to offer several advantages over the conventional complement fixation and proteus agglutination tests. During the coming year these procedures will be applied to epidemiologic and clinical characterization of spotted fever in North Carolina.